Fix null-safety issue with mapBounds in getNearbyNodes

Change mapBounds from LatLngBounds? to final LatLngBounds so the
compiler can prove it's non-null after the inner try-catch. Addresses
review comment on PR #45.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/widgets/map_view.dart

@@ -160,7 +160,7 @@ class MapViewState extends State<MapView> {
       getNearbyNodes: () {
         if (mounted) {
           try {
-            LatLngBounds? mapBounds;
+            final LatLngBounds mapBounds;
             try {
               mapBounds = _controller.mapController.camera.visibleBounds;
             } catch (_) {